Docs: improve RedwoodRecord docs

[olance]

After using RedwoodRecord a little, I found there were some errors and some improvement opportunities in the current docs:

• A lot of the code snippets had missing await; there was also some JS syntax errors
• find is documented to raise when no record is found, but it was not explicitly said what the behaviour of findBy/first is.
• Validation code sample used User.isValid() (as a method) when it's actually a property: User.isValid

[cannikin]

Thank you sir!